The instant invention relates to chemical vapor deposition (CVD) apparatus and methods, and more particularly to a parylene deposition apparatus incuding a dry vacuum pump system connected directly to the deposition chamber, and further including a cold trap located downstream of the vacuum pump.
Parylene is a general term used to describe a class of poly-p-xylylenes which are derived from a dimer having the structure: ##STR1## wherein X is typically a hydrogen, or a halogen. The most commonly used forms of parylene dimers include the following: ##STR2##
Parylene coatings are obtained from their related parylene dimers by means of a well-known vapor deposition process in which the dimer is vaporized, pyrolized, i.e. cleaved into a monomer vapor form, and fed to a deposition chamber wherein the monomer molecules deposit and polymerize onto a substrate disposed within the deposition chamber. The process occurs according to the following reaction: ##STR3##
Due to their ability to provide thin films and conform to substrates of varied geometric shapes, parylene polymers are ideally suited for use as a conformal external coating in a wide variety of fields, such as for example, in the electronics, automotive, and medical industries.
Octafluoro-[2,2]paracyclophane (Parylene AF4 dimer) is a fluorine substituted version of the above-noted parylenes and has the structure: ##STR4##
It is known that parylene coatings which are derived from the AF4 dimer by the vapor deposition process have a very high melting temperature (about 500.degree. C.) and a very low dielectric constant (about 2.3). These characteristics make Parylene AF4 ideally suited for many high temperature applications, including electronic applications, and potentially as an inter-layer dielectric material in the production of semiconductor chips. The existing parylene coating systems as used with Parylene C, D, and N, typically include a chamber system comprising a vaporization chamber, a pyrolysis chamber coupled to the vaporization chamber, and a deposition chamber coupled to the pyrolysis chamber in which the monomer vapor deposits onto a substrate and polymerizes. The existing coating systems further include a vacuum system coupled to the chambers for creating sub-atmospheric pressure conditions throughout the chamber system. Typically, a liquid nitrogen cold trap is connected to the deposition chamber, and a vacuum pump is located downstream of the cold trap. The primary purpose of placing the cold trap between the deposition chamber and the vacuum pump was to trap any excess monomer and prevent the monomer from flowing through the pump where it could deposit internally and interfere with operation of the pump, and/or contaminate the pump oil. The prior Parylene materials (Parylene C, D, and N) effectively deposited onto surfaces having a temperature of about 15.degree.-40.degree. C. which just happens to be the normal operating temperatures of such pumps. The cold trap was further effective for preventing backstreaming of oil vapors into the deposition chamber. Hence, the cold trap was necessary to trap excess monomer from depositing onto the internal surfaces of the vacuum pump, and prevent oil vapor from backstreaming into the deposition chamber.
However, placement of the cold trap in direct communication with the deposition chamber is known to cause at least one drawback. It has been found that the cold trap creates a cryo-pumping arrangement which has been shown to rapidly draw monomer through the system faster than the background atmosphere is pumped by the vacuum pump. The result is that the monomer is drawn very quickly through the deposition chamber, and is not allowed sufficient time to polymerize. The problem has been compensated for by utilizing excess dimer, and increasing coating cycle time. This was not previously found to be that significant a drawback in that the cost of Parylene C, D, and N is not prohibitive.
While the existing parylene deposition systems are highly effective in depositing parylene C, D, and N, there are unique characteristics of the AF4 molecule which prevent the existing parylene coating systems from providing sufficient deposition control, uniformity of layer thickness, material efficiency, and speed of coating to be compatible with existing semiconductor chip manufacturing technologies, semiconductor chip cost structures, and semiconductor chip manufacturing time constraints. Accordingly, there is currently presented a need for a parylene deposition system particularly suited to depositing parylene AF4 polymer onto semiconductor wafers for use in the production of semiconductor chips.
The instant invention provides a parylene deposition system comprising a vaporization chamber, a pyrolysis chamber, a deposition chamber, and a vacuum system wherein an oilless, dry vacuum pump is connected directly to the deposition chamber and the cold trap is located downstream of the vacuum pump. More specifically, the outlet of the vacuum pump is coupled to an inlet of the cold trap which is preferably cooled by means of liquid nitrogen received in a cooling finger disposed in the cold trap. The outlet of the cold trap opens to ambient atmosphere. In operation, any free monomer which did not deposit in the deposition chamber passes through the vacuum pump and deposits onto the surface of the cold finger disposed in the cold trap chamber. It has been found that the Parylene AF4 monomer will not deposit onto warm substrates and thus no longer poses the problems of deposition on the inner surfaces of the vacuum pump.
Accordingly, among the objects of the instant invention are: the provision of a parylene deposition apparatus effective for quick and efficient deposition of Parylene AF4 onto silicon wafers in the production of semiconductor chips; the provision of a parylene deposition apparatus including a vacuum system wherein an oilless dry vaccum pump is connected directly to the deposition chamber, and a liquid nitrogen cooled cold trap is connected to the outlet of the vacuum pmp; and the provision of further means for the fast, efficient, and cost effective deposition of Parylene Af4 onto the surface of a silicon wafer.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.